1. Field of the Invention
This invention relates generally to dilatometers and, more particularly, to dilatometers having a linear variable differential transformer dilation sensor with an improved independent support mechanism for the core or coil or both which permits the maintenance of a constant load on the test specimen and facilitates accurate measurements on a specimen which is subject to large dimensional changes with changes in temperature.
2. Description of the Prior Art
Dilatometers are analytical instruments that respond to the linear thermal expansion or contraction of solids. Typically, these instruments have a variable temperature furnace in which the test specimen rests between a flat surface on a stationary object and an opposing flat surface on a movable object, such as a ceramic pushrod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a mechanical, optical or electrical system for amplifying and measuring that change. These instruments can be used to make precise measurements of changes in length resulting from small temperature changes or to plot variations in the rate of linear expansion or contraction over a broad temperature range.
U.S. Pat. Nos. 3,680,357 and 3,898,836 describe a particularly accurate type of dilatometer in which the dilation sensor is a linear variable differential transformer in which the core floats freely in the coil and each of these elements is separately supported at its ends by a pair of compound cantilevered springs. These springs permit independent and frictionless axial movement of the suspended element, but restrain radial or transverse movement. This independent and frictionless axial movement of the core and coil facilitates calibration of the linear variable transformer and, when core and coil are coupled to separate pushrods, it makes possible exceptionally accurate differential measurements of two different specimen materials.
Such differential measurements are invaluable in studying the compatibility under changing temperature conditions of different materials which are bonded together or are in close contact; e.g., metal to glass, enamel to substrates, thin film deposits in microcircuits or metal or plastic fillings in natural teeth. When the coil and core pushrods are of the same material and configuration and they extend the same distance into the furnace, the linear variable differential transformer automatically compensates for dilation of those pushrods. Two pushrod dilatometers of this type also are useful for making measurements on a single specimen. When one pushrod abuts the specimen and the other abuts the fixed specimen support adjacent the specimen, the dilation of the fixed support likewise is cancelled by the linear variable differential transformer.
The ability of a dilatometer to compensate automatically for pushrod and fixed specimen support dilation is especially important in applications where a massive fixed specimen support is essential or large specimen dilation necessitates the use of unusually long pushrods which extend deeply into the furnace. Exemplary of such applications is the use of a two rod dilatometer in simulating commercial processes, such as firing of ceramics or sintering of either metallic or non-metallic powders, and in determining the softening point of glass by parallel plate viscometry.
In simulating the firing of ceramics or the sintering of powders, one pushrod typically abuts the fixed specimen support and the element of the linear variable differential transformer to which that pushrod is coupled is subjected to relatively small axial displacement. The other element, however, is coupled to the pushrod that abuts a specimen that shrinks significantly under the influence of heat and this element is subjected to much greater axial displacement. While the requirement for this large axial displacement (i.e., long range) can be accommodated by supporting that element on larger than normal compound cantilevered springs, such large springs add undesirable mass to the dilatometer measuring head and often may impose an undesirably large load on the specimen.
In order to overcome these shortcomings, a two pushrod dilatometer has been developed in which a short range coil of the horizontally oriented linear variable differential transformer is supported by a pair of conventional compound cantilevered springs, and a long range core is provided by supporting that core on linear ball bearings within tubular housings that are welded to the ends of the coil and that encircle the protruding ends of the core. A small constant load on the specimen is provided by a weight attached to a cord that passes over a pulley and connects to the core, thereby urging the core and its pushrod toward the specimen.
While this two pushrod dilatometer design provided the long range and small constant specimen loading that are desirable for handling specimens that shrink significantly, accurate alignment of both elements of the linear variable differential transformer and their coupled pushrods with the specimen and fixed specimen support proved to be an exceedingly tedious operation. This was due to the fact that the core and coil were laterally locked together and the weight of both, as well as that of both pushrods, was carried by the pair of springs attached to the coil.
Lateral movement of the coil for alignment purposes could be accomplished by making small adjustments in the points of attachment of the single pair of springs to the dilatometer base (as was commonly done with dilatometers having two pairs of springs), but it was an awkward operation because of the necessity of manually holding the entire unwieldy assembly of coil, coil springs, core and two pushrods in proper position while changing the points of spring attachment and securing those springs to the dilatometer base. Independent lateral movement of the core was not possible.
In addition, the laterally locked together coil and core necessitated an unusual degree of precision in alignment, particularly when it was desirable to impose a large constant load on the specimen through the long range core. Unless the core, its coupled pushrod and the specimen were accurately aligned, the lateral vector of a heavy loading force might exceed the capacity of the single pair of springs to limit lateral displacement of the entire assembly of core, coil and two pushrods.